What is the difference between a used LY6 6.0 and an L92 6.2, and why does the L92 cost twice as much?

They're closer than the price gap suggests. Both are Gen IV engines with VVT and rectangle-port heads.

Engine	Size	Years	Power	Torque	Compression	Block	AFM
LY6	6.0 L	2007–2013	361 hp	385 lb-ft	9.60:1	iron	No
L92	6.2 L	2007–2008	403 hp	415–417 lb-ft	10.50:1	aluminum	No

The differences in our data: the L92 is 6.2 L with an aluminum block and 10.50:1 compression. The LY6 is 6.0 L with an iron block and 9.60:1; our data notes the LY6 uses L92/LS3-style rectangle-port heads without sodium-filled exhaust valves. Stock, that's 403 vs 361 hp.

Price: we don't have a source on used-engine pricing. The L92's aluminum block and extra displacement are the obvious spec differences; our data lists it for 2007–2008. Check current prices with the donor finder eBay links.